Method for precipitating or flocculating substances out of solutions

ABSTRACT

In a method for precipitating or flocculating substances out of a solution, the solution is brought into contact with at least one ion exchange material having a surface provided with functional groups loaded with counter ions. The precipitation or flocculation is effected catalytically without exchange of the counter ions for ions contained in the solution.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for precipitating or flocculatingsubstances out of solutions.

2. Discussion of the Relevant Art

Undesirable ionic substances contained in water can be removed when theyare transformed into the form of a sparingly soluble salt or mineral andare thus precipitated. Many metal ions, such as, for example, Ca²⁺,Mg²⁺, Fe²⁺, Me²⁺ ions can be precipitated in the form of sparinglysoluble hydroxides. Such reactions can be controlled via the pH value.

Ca²⁺ ions in water are removed on a commercial scale in that they areprecipitated as CaCO₃ (calcium carbonate) (decarbonization) Thisreaction is also controlled via the pH value.

Closely related to the precipitation of substances contained in water isthe term of flocculation and sedimentation. This is so because theremoval of (precipitated) substances contained in water requires:thatthey can also be separated from the water. In the context offlocculation and sedimentation it is important how the precipitatedproducts grow further and/or can conglomerate. The addition of certainsalts (aluminum salts, iron salt) can control this behavior.

In the conventional method technology it is difficult to avoid a localoverdosage when introducing the flocculation agent (for example, whenadding sodium hydroxide solution or when dissolving sodium hydroxidepellets). A local overdosage can result in the precipitation ofinherently less soluble substances contained in the water which thencause, as an entrained solid particle, component hard-to-controlconditions in the subsequent precipitation process.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved method forprecipitation or flocculation of substances contained in solutions,especially water.

According to the invention, the method is characterized in that thesolution is brought into contact with at least one ion exchange materialwhich releases ions into the solution that effect precipitation orflocculation and/or has on its surface functional groups whichcatalytically effect flocculation or precipitation.

Ion exchange materials have been used in water or sewage treatment inorder to exchange undesirable ions against desirable ions or ions thatare less disruptive for the respective application purposes. Known are,for example, water softening devices which by means of ion exchangersbind Ca²⁺ and/or Mg²⁺ in exchange for Na²⁺ or H⁺ ions. Anionicexchangers (mostly in the Cl⁻ or OH⁻ form) allow the removal ofundesirable anions (NO³⁻, HCO³⁻ etc.) from the water. Known are alsomethods in which the Cu²⁺ ion or heavy-metal ions are removed by meansof ion exchangers from the water. All these methods have in common thatthe ions removed from the water are bonded to the resin; once thecapacity of the resin is depleted, it must be regenerated. During theregeneration process, the metal ions which have been concentrated canbe, for example, removed from the regenerated compound.

Novel is now the idea to employ an ion exchange material for inducing aprecipitation or flocculation process.

According to a first aspect of the invention, ion exchange materials areused as supports for the ions which in solution make the precipitationreaction possible. The component required for transformation of theionic species to be precipitated into a sparingly soluble salt/mineralis provided by the ion exchange material which has been conditioned forthis purpose. For example, an ion exchange resin of the OH⁻ formprovides the required OH⁻ ions for a hydroxide precipitation in orderto, for example, precipitate Fe²⁺ and Mn²⁺ in the form of hydroxides butof the water. Ag⁺ ions in the water can be precipitated as AgCl in thepresence of an anionic exchange resin of the Cl⁻ form.

When using specially conditioned ion exchange materials, the followingadvantages will result relative to the prior art in the field of watertreatment:

The ion exchange material allows the directed addition of the componentsrequired alone for the precipitation reaction, for example, for thedecarbonization of calcium carbonate-containing water. The principle ofdecarbonization of calcium carbonate-containing water is that the pHvalue is to be raised in order to shift the calcium carbonate/carbonicacid equilibrium such that the Ca²⁺ ions will precipitate in the form ofcalcium carbonate. Conventionally, the pH value increase is achieved byadding Ca(OH)₂ NaOH and/or NaCO₃. This addition has the disadvantagethat with the OH⁻ or CO₃ ²⁻ ions acting as a base, additionally Ca²⁺ orNa³⁰ ions are introduced into the water which partially counteract(additional Ca² + ions which must be precipitated) or limit (sodiumlimit value in drinking water) the success of the method.

A weakly basic ion exchange resin of the OH⁻ form only releases OH⁻ions; an ion exchange resin of the HCO³⁻/CO₃ ²⁻ releases only CO₃ ²⁻ andHCO³⁻ ions.

Better control of the precipitation process by avoiding localoverdosage, especially in combination with a fluidized bed variant.

The possibility of controlling the precipitation process by means of thecontact time of the water to be treated with the ion exchange material.

The ion exchange is a surface process and depends on the degree ofloading of the ion exchange material with the ions required for thereaction and the type and concentration of ions in the solution whichcan be exchanged for the ions on the resin.

The contact time can be adjusted simply and can be changed optionally(by the size of the ion exchange resin bed and the flow-through amountin continuous operation; via the residence time in the reaction vessel(tank) during batch operation).

The recyclability of the ion exchange material.

Depleted ion exchange material, especially resins, can be removed easilyfrom the reaction vessel or tank and regenerated. The regeneratedmaterial can then be returned into the process.

Ion exchange materials can be used as carriers of ions which control theflocculation in solution. In analogy to the above described mechanism,ions which enhance the flocculation of substances contained in the water(for example, Al³⁺ and Fe³⁺ ions) can also be brought into thecorresponding solution by ion exchange from an ion exchange material (itis then required to provide an ion exchange material that is at leastpartially loaded with Al³⁺ and Fe³⁺ ions). All advantages are alsoapplicable here.

For certain processes, the combination of dosage of pH-controlling ions(for example, anionic exchangers of the OH⁻ form) and flocculationagents (for example, cationic exchangers in the Fe²⁺ form) is expedient.

According to a second aspect of the invention, a specially conditionedion exchange material can be used as a catalyst for precipitation ofsubstances contained in water. In many real solutions there is thesituation that the solution, when considered thermodynamically, isoversaturated with respect to a dissolved phase. Despite this fact,within a finite time period no precipitation takes place which wouldbring the solution into equilibrium. Such meta-stable solutions lacksuitable growth locations where the precipitation could take place.Suitable growth locations are crystal seeds of the phase to beprecipitated or special heterogeneous surfaces which decreaseconsiderably the seed formation work and thus make the formation ofcrystal seeds in the range of low saturations possible. An example forsuch a solution is water which is oversaturated with respect to calciumcarbonate.

It is known that biological systems (muscles, algae etc.) are able toinitiate a directed crystal seed formation by means of certainfunctional groups. In particular, it was found that the carboxyl groupof certain carboxylic acids (stearic acid etc.) induces calciumcarbonate crystal seed formation. In regard to a mechanism of thisreaction, it is assumed that carboxyl groups first bind Ca²⁺ ions fromthe water and that only this combination is able to induce the calciumcarbonate crystal seed formation.

Ion exchange materials obtain their specific properties also as a resultof certain functional groups: strongly acidic ion exchange materialscarry as active functional groups, for example, the sulfonate group;weakly acidic ion exchangers have as active functional groups, forexample, the carboxyl group (COO⁻).

When the carboxyl group of a weakly acidic ion exchange material isloaded by means of a loading process preferably completely with Ca²⁺ions, this loaded material is suitable to catalytically form CaCO₃crystal seeds on its surface in aqueous calcium carbonate-containingsolutions.

Such a conditioned weakly acidic ion exchange material can be used, forexample, as a nucleus-forming agent and filter pellet in conventionaldecarbonization devices; and, furthermore, for the increase of the seedformation rate and thus the efficiency in the method and devicedescribed in the German patent application DE 19606633 A1. The contentsof DE 19606633 A1 is included in the disclosure of the presentapplication.

The catalytic efficiency depends on the bonding strength (electrostaticassociation) between carboxyl group and the Ca²⁺ ion: a bond which istoo strong would not make possible the association of carbonate ionsfrom the solution required for the seed formation; a bonding that is tooloose would result in the loss of Ca²⁺ and thus in the destruction ofthe catalytic complexes. The electrostatic association of carboxylgroups and Ca²⁺ ions on the interface ion exchange material/water isaffected by the electrical field on the interface. The catalyticefficiency of these specially loaded ion exchange materials isaccordingly increased when they are, for example, applied to theelectrodes described in the international application WO 95/26931 orproduces them therefrom and, in this way, modulates or adjusts thefunctional groups by means of the described intrinsic electrical field.The contents of WO 95/26 931 thus is included in the disclosure of thepresent application.

EMBODIMENTS

The decarbonization of calcium carbonate-containing water via thedirected dosage of OH⁻ ions via an ion exchange resin:

On a commercial scale, the decarbonization of calciumcarbonate-containing water has been realized in that, by addition ofcertain chemicals (milk of lime, sodium hydroxide, soda), the pH valueof the water was raised and thus the calcium carbonate/carbonic acidequilibrium was shifted greatly toward oversaturation. The resultinghomogenous seed formation generated calcium carbonate crystal seeds onwhich the calcium carbonate dissolved in water then would precipitate(Mg²⁺ ions precipitate as Mg(OH)₂).

The success of the method depends greatly on the type of processcontrol.

The use of sodium hydroxide for increasing the pH value is a problembecause at the location of addition of the sodium hydroxide an extremepH value increase results locally which causes the precipitation ofundesirable hydroxides. These hydroxides, for example, Ca(OH)₂ colloids,are entrained as solid bodies into the process water and make therequired pH value adjustment after the decarbonization process moredifficult.

The problem when using milk of lime (Ca(OH)₂) lies in the preparation ofthe solution to be added and the addition: it is practically impossibleto produce a dosage solution which is free of Ca(OH)₂ colloids. Whenthese colloids are not completely dissolved in the decarbonization step,they present a great problem in the pH value reduction requiredsubsequently.

The addition of milk of lime also adds further Ca²⁺ ions to the waterwhich in the subsequently precipitation process are only partiallyprecipitated also. Often, additional carbonate (in the form ofsoda-Na₂CO₃) must be added in order to be able to satisfactorily removeCa²⁺ ions by precipitation. However, this also results in theundesirable increase of the Na⁺ contents in the water.

The goal of an optimal process control is furthermore a controlled seedformation rate: too many crystal seeds compete in regard to their growthand grow only to small calcium carbonate crystals which can be separatedonly with difficulty from the process water (sedimentation speed is toolow, filtration is complex).

The use of a (strongly basic) anionic exchanger (for example, of the OH⁻form) makes it possible to have an optimal process control.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIG. 1 a schematic of a decarbonization device that operates on thebasis of strongly basic anionic exchangers is illustrated.

FIG. 2 shows a device for producing seed crystals.

FIGS. 3 and 4 show further embodiments of a device suitable forperforming the method according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the decarbonization stage 1 of FIG. 1 the raw water flows via theline 2 first through a bed of strongly basic ion exchange material 3; bymeans of ion exchange the pH value of the water is adjusted to a pHvalue between 9 and 10 (depending on decarbonization efficiency). Thecontrol of the pH value. is carried out by means of the average contacttime of the raw water with the ion exchange material 3 (flow-through,resin amount). By raising the pH value, a homogeneous seed formationresults, and, accordingly, calcium carbonate precipitation is caused.The precipitated calcium carbonate is separated in the following filterstage 4 according to the prior art by sedimentation and/or filtrationfrom the process water (sedimentation filter 6). In the third stage 5the pH value is optionally adjusted.

In order to make possible the continuous operation of such a device, itis expedient to regenerate the ion exchange material continuously. Thisis carried out best in that a part of the partially spent material 3 isremoved from the bed, for example, by a vacuum and supply line 7. Theremoved amount is replaced by a corresponding amount of freshlyregenerated ion exchange material 3, is regenerated (regeneration iscarried out in the regeneration device 8, for example, with acids orelectrolytically, wherein at the same time disinfection and washing canoccur) and is then available for further use.

Expediently, the device (stage 1) is operated by fluidized bedoperation. With the ion exchange material supply and removal device,used ion exchange material is removed periodically or continuously fromthe fluidized bed and replaced by fresh resin. Cleaning is possible bymeans of backwash lines 9 and the flushing outlets 10.

In order to raise for hard water with a total hardness of 4 mmol/l, aDIC contents of also 4 mmol/l, and a pH value of 8.0, the pH value toapproximately 9.5, approximately 1 mmol/l base or OH⁻ ions are required.A strongly basic ion exchanger of the OH⁻ form, for example, Lewait MP600 of the firm Bayer/5/, has a capacity of typically 1 val/l, i.e.,based on the requirement of 1 mmol/l OH⁻ ions per liter raw water,approximately 1000 liters can be correspondingly treated per literresin.

The resin amount required for a certain treatment efficiency depends onthe contact time resin/water required for the pH value increase. In theabove described water, a contact time of approximately 30 seconds issufficient for a pH value increase to 9.5. For a decarbonization devicewith an output of 100 m³/h this results in a required resin amount ofapproximately 850 liters.

Iron Removal, Demagnetization of Water

In a similar manner the reduction of iron and manganese ions from thewater can be carried out. For this purpose, preferably a weakly basicanionic exchanger material, for example, Lewait MP62 of the firm Bayer,is used because, generally, only a medium pH value increase is requiredin order to initiate the precipitation of iron and manganese hydroxides.

In order to improve flocculation of the hydroxides, it is beneficial toadd in minimal amounts to the resin bed a resin which is loaded withaluminum ions or complexes (for example, a strongly acidic ion exchangeresin Lewait S 100 of the firm Bayer loaded with aluminum ions).

Catalytic Precipitation of Calcium Carbonate

A weakly acidic ion exchange material preferably completely loaded withCa²⁺ ions, for example, a resin Lewait CNP 80 of the firm Bayer,triggers in calcium carbonate-containing solutions catalytically calciumcarbonate crystal seed formation.

The latter resin can be used, for example, in order to enhance or toreplace the above described decarbonization. In the process control ofdecarbonization it is favorable not to allow the pH value to become toohigh in order to maintain a crystal seed concentration that is not toogreat. A pH value that is too great increases also the expenditure ofthe subsequent pH value reduction. A crystal seed density that is toohigh results in many small calcium carbonate crystals which can beseparated only with difficulty from the water. A weakly acidic resin ofthe Ca²⁺ form forms crystal seeds also for low oversaturation. With theuse of this resin it is possible to control the process such that the pHvalues must not be controlled above the pH value 9 so that theoversaturation remains in a range in which no sudden high crystal seeddensity is generated.

The converted CNP 80 can be dried, ground, and applied as a thin layeronto a support, for example, the electrodes disclosed in theinternational application WO 95/26931. With the intrinsic field of sucha coated electrode, the catalytic activity of the functional groups canbe controlled. In this manner, the directed crystal seed formation canbe initiated. This effect can be used in water treatment in order tosupply a defined amount of calcium carbonate crystal seeds to theprocess water.

In this context, the invention is in no way limited to known ionexchange materials. It is only important that the employed material cancarry active groups which are able to receive ions from the solution andto release others instead. These groups therefore must have a finitedissociation constant in the liquid in question.

In order for the materials to have catalytic properties, it isadditionally advantageous when those materials are used which have amicrostructure favorable for the crystallization. This is, for example,the case when the basic matrix onto which the groups are applied, is atwo-dimensional template which has a good conformity with the latticeconstant of the crystal to be formed so that electrostatic andstereochemical conditions as in the crystal to be formed are present.The active groups are then to be prepared such that at least an ioniccomponent of the substance to be crystallized is absorbed. It is thenable in the oversaturated solution to initiate crystal seed formation onthe interface.

In this respect, suitable materials (matrix or support materials) arepreferably polyacrylate, polystyrene, activated carbon (as granules orporous semi-finished parts in the form of disks, cylinders, hollowcylinders) which can be functionalized preferably with a carboxyl group.The carboxyl group is usually saturated during the manufacturing processin the H⁺ form. In order to use this material, for example, for thecatalytic crystallization of calcium carbonate, the H⁺ ions are replacedby cations of sparingly soluble salts (for example, Ca², Mg²⁺, Fe²⁺,Cu²⁺ etc.) so that in the end an ion exchange material in the respectivecationic form is present (Ca²⁺ form, Mg²⁺ form, Fe²⁺ form, Cu²⁺ formetc.). The geometric position of the Ca²⁺ ions on the surface of apolyacrylate resin ball of the weakly acidic ion exchange resin LewatitCNP80 of the firm Bayer is determined by the molecular geometry of thepolyacrylate matrix. The thus produced surface now exhibits goodelectrostatic and stereochemical properties for the formation of CaCO₃crystals.

As already mentioned above, the electrostatic and stereochemicalproperties are important for the catalytically induced formation ofcrystal seeds on the respective surface. The stereochemical andsubstantially also electrical properties are adjusted via the structureof the basic material (for example, polyacrylate) on which the activegroups are seated.

The electrostatic properties can be affected additionally by an externalelectrostatic field. In a simple way, this can be realized byintroducing the catalyst material between two field-generatingelectrodes. As a concrete realization for this purpose, the containerwall (for example, of a fluidized bed reactor) can be switched as acathode and an anode can be positioned centrally within the tank.

However, an especially elegant variant results when the catalystmaterial is applied as a thin layer on an electrode, as is described,for example, in the international application WO 95/26931, and theelectrostatic properties of the catalytic boundaries are adjusted bymeans of an intrinsic field.

FURTHER APPLICATION EXAMPLES Example 1

Such a catalyst can be used for the formation of seed crystals which aredistributed by the water flow in the installation and pipeline systemdownstream. Accordingly, seed crystals thus are formed as theprecipitation product. It is known that such crystal seeds can preventby their growth process the deposition on pipe walls or heat registersof hot water heaters. For the protection of a drinking waterinstallation in a household, it is, for example, possible to use afluidized bed reactor 11 (volume approximately 6 to 8 liters, diameter15 cm, height 60 cm) with a catalyst filling (for example, four liters).The catalyst bed 12 is, for example, formed by a weakly acidic cationicexchanger of the Ca²⁺ form (Lewait CNP90 of the firm Bayer). The rawwater flows from the inlet 13 via a pump 14 and a jet bottom 15 as wellas a support layer 16 of quartz sand through the catalyst 12. By meansof the pump 14 the catalyst bed is permanently fluidized (circulation)via the check valve 18 and the pump 14. The constant flow and frictionof the catalyst granules prevents blockage of the granules andadditionally enhances the detachment of the crystal seeds from thecatalyst surface. The crystal seeds are carried out by the removal ofwater (line 17) as crystallization seeds into the attached installationsystem.

Example 2

A catalytically active material prepared such is especially suitable asa bottom deposit for a method for treatment of water as disclosed inGerman patent DE 19606633 A1.

In the decarbonization of drinking water with high calcium carbonatecontents by means of pH value increase with Ca(OH)₂ (typical pHvalues>12), the following method disadvantages are known:

High turbid substance contents in the overflow water of the reactor andthus the necessity of a filtration stage downstream.

High pH value of the product water must be lowered with great expense.

High use of chemicals.

By using a catalytically active material (for example, weakly acidiccationic exchanger of the Ca²⁺ form) in the reactor, the above-mentioneddisadvantages can be practically completely prevented. The pH value mustonly be raised minimally (to a maximum of 9) in order to generate at thecatalytic surfaces a sufficient seed formation. Accordingly, primarilychemicals, a filtration stage and neutralization stage can be saved.

In the embodiments illustrated in FIGS. 3 and 4, the water treatment bymeans of a catalytically active ion exchange material 12, which isarranged in a container 11, is combined with a preferably physical watertreatment device 19. Such a physical water treatment device can operate,for example, electrostatically. Water treatment devices as they aredescribed in the international application WO 95/to 6931 and the Germanpatent application DE 19606633 are especially suitable.

Advantageosuly, the water to be treated, especially for decalcification,is guided by pump 14 in circulation through the ion exchange material.The water treatment device 19 can either be mounted within this circuit(FIG. 3) or can be arranged downstream of this circuit (FIG. 4).

What is claimed is:
 1. A method for precipitating or flocculating asubstance out of a solution, the method comprising the steps of:providing at least one ion exchange material having a surface providedwith functional groups loaded with counter ions; bringing a solutioninto contact with the at least one ion exchange material; and effectingprecipitation or flocculation of a substance catalytically withoutexchange of the counter ions for ions contained in the solution.
 2. Themethod according to claim 1, wherein the counter ions are cations. 3.The method according to claim 2, wherein the counter ions are selectedfrom the group consisting of Ca²⁺, Fe²⁺, and Cu²⁺.
 4. The methodaccording to claim 1, wherein the counter ions comprise at least oneionic component of the substance to be precipitated or flocculated. 5.The method according to claim 4, wherein, when the solution is water,the counter ions are Ca²⁺ ions for forming calcium carbonate crystalseeds.
 6. The method according to claim 5, wherein the ion exchangematerial is weakly acidic.
 7. The method according to claim 6, whereinthe functional groups are carboxyl groups (COO⁻) carrying the counterions.
 8. The method according to claims 7, wherein the counter ions areCa2+.
 9. The method according to claim 1, further comprising the step ofexposing the surface of the ion exchange material in contact with thesolution to an electrical field.
 10. The method according to claim 9,wherein the electrical field is adjustable.
 11. The method according toclaim 9, further comprising the step of arranging electricalfield-generating electrodes in or on the ion exchange material such thatthe solution is not present between the electrical field-generatingelectrodes.
 12. The method according to claim 1, further comprising thestep of circulating the solution along the ion exchange material orthrough the ion exchange material.
 13. The method according to claim 1,further comprising the step of passing the solution along or through theion exchange material and guiding the solution additionally through aphysical water treatment device.
 14. The method according to claim 1,wherein the solution to be treated is water.
 15. The method according toclaim 1, wherein the ion exchange material is comprised of polyacrylate,polystyrene or active carbon and wherein a surface of the polyacrylate,polystyrene or active carbon carries the functional groups.
 16. Themethod according to claim 1, further comprising the step of removing theprecipitated or flocculated substance from the solution.
 17. The methodaccording to claim 1, wherein the precipitated or flocculated substanceforms seed crystals.